1. Field of the Invention
The invention relates in general to a wireless communication system and associated method, and more particularly to a transceiver and a noise cancellation method for radio-frequency identification (RFID).
2. Description of the Related Art
RFID is considered as one of top ten most influential techniques in the twenty-first century. Referring to FIG. 1, an RFID system generally requires a reader and an RFID tag. In an RFID operation, an RF electric wave is transmitted by the reader 10 to trigger the RFID tag 12 within coverage, and an electric current is generated through electromagnetic sensing to power a chip on the RFID tag 12 and to backscatter a wireless signal to the reader 10. Based on driving energy, RFID tags are categorized into active and passive types. A passive tag is not equipped with a battery and is powered by electric energy generated through electromagnetic sensing the RF electric wave of the reader, inferring that the passive tag only passively backscatters to a reader upon receiving a signal transmitted by the reader. In contrast, an active tag is equipped with a battery, and is thus capable of transmitting a signal to be read by a reader and has a transmission range broader than that of a passive tag.
The RFID tag 12 usually transmits a message via a modulated carrier signal when backscattering to the reader 10; however the reader 10 at this point still transmits unmodulated carrier signals for powering the passive tag. FIG. 2 shows a structure in the reader 10. Majority of a carrier signal Cx sent by the transmitter 14 is transmitted to the environment. Due to slight impedance mismatch in real situations, a small part of the carrier signal Cx is reflected by the antenna 18, as indicated by a reflected carrier signal CRx in FIG. 2. The reflected carrier signal CRx and a wireless signal Rx received by the antenna 18 are jointly received by the receiver 16 via a coupler 20. Compared to the desired wireless signal Rx, the reflected carrier signal CRx is equivalently noises that should be restrained or eliminated.
FIG. 3 shows a spectrum of the reflected carrier signal CRx and the wireless signal Rx. It is extremely difficult for the reader 10 to send an absolutely clean (monotone) carrier signal Cx, as the carrier signal Cx more or less contains a phase noise. Therefore, the spectrum of the reflected carrier signal CRx is spread regarding a carrier frequency fCx as a center. In a modulated result, the wireless signal Rx is substantially consisted of two tones (having frequencies of subtracting/adding a frequency difference Δf from/to the carrier frequency fCx of the carrier signal Cx), as shown in FIG. 3. The presence of the reflected carrier signal CRx reduces a signal-to-noise ratio of the receiver 16 at a receiving terminal. Referring to FIG. 3, once the reflected carrier signal CRx is aggravated, the wireless signal Rx may be overwhelmed by the reflected carrier signal CRx and become unidentifiable.
Therefore, there is a need for a solution for eliminating or restraining the reflected carrier signal CRx. A most instinctive approach is to decrease the impedance mismatch to directly reduce the energy of the reflected carrier signal CRx. However, such approach requires high-precision impedance matching and thus significantly increases costs for manufacturing readers.